The present invention relates to interferometers of the type to be used for detecting surface defects and other deviations of the surface contour of a mirror from the desired contour as well as inhomogeities in a medium.
The art of interferometers is fairly well established. The "text book" variety of interferometers goes back, basically to Michelsen and consists of a light beam, a beam splitter directing components of the beam toward two mirrors and recombining the two reflected beams to obtain an interference pattern which can be used for a variety of purposes including the detection of flaws in one mirror while using the other one as a reference.
This kind of interferometer produces "fringes" which are lines of similar optical path differences (as between the test beam and the reference beam). In the case of reflecting surfaces the different fringes or fringe lines delineate the contours of the level difference between portions of the reflecting surfaces in units of .lambda./2 or .lambda., where .lambda. is the wave-length of the radiation used. The resolution of this method of determining the contour on the basis of optical path difference is, therefore, limited by the chosen wavelength. The present invention relates to an interferometer which still employs the basic principle of beam splitting, separate reflections on test and reference mirrors, and recombining the two reflected components for purposes of interference. However, the invention is directed towards an attempt to improve the resolution, specifically by improving the detection resolution of optical path difference measurements.
Other types of interferometers are known such as the Fabry Perot interferometer, or the Twyman Green interferometer which really is an outgrowth of the Michelson device. Aside from the restriction as to resolution outlined above, the known interferometers suffer from errors on account of vibrations of and in the equipment. Furthermore, there is a need to acquire electrical signals in a manner which permits the direct and immediate generation of data representing optical path differences. Moreover the electrical signals will be processed for purposes of further analysis of surface quality, homogeneity, etc; including the generation or contour maps or the like.